Directional sensitive fiber optic cable wellbore system

ABSTRACT

A fiber optic cable assembly includes an elongate housing, a signal fiber placed inside the housing and extending longitudinally, and a plurality of sensing fibers placed inside the housing and extending longitudinally. The plurality of sensing fibers is placed around the signal fiber. Each of the plurality of sensing fibers carries a respective laser signal of a distinct frequency. The signal fiber carries one or more evanescent coupling signals responsive to the laser signals in the plurality of sensing fibers.

CLAIM OF PRIORITY

This application claims priority to and is a continuation of U.S. patentapplication Ser. No. 15/864,284, filed on Jan. 8, 2018, the entirecontents of which is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a fiber optic cable system used in awellbore.

BACKGROUND

Fiber optic cables are used today for downhole sensing in a wellbore.For example, distributed acoustic sensing (DAS) systems and distributedtemperature sensing (DTS) systems use fiber optic cables to measuretemperatures and detect acoustic frequency strain signals in thewellbore, respectively.

SUMMARY

This disclosure relates to a directional sensitive fiber optic cablewellbore system.

In an implementation, a fiber optic cable assembly includes an elongatehousing, a signal fiber placed inside the housing and extendinglongitudinally, and a plurality of sensing fibers placed inside thehousing and extending longitudinally. The plurality of sensing fibers isplaced around the signal fiber. Each of the plurality of sensing fiberscarries a respective laser signal of a distinct frequency. The signalfiber carries one or more evanescent coupling signals responsive to thelaser signals in the plurality of sensing fibers.

The details of one or more implementations of the subject matterdescribed in this disclosure are set forth in the accompanying drawingsand the description later. Other features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 2 illustrates a second cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 3 illustrates an example physical dimension of a schematic of adirectional sensitive fiber optic cable assembly for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIGS. 4A-4C illustrate cross sections of schematics of exampledirectional sensitive fiber optic cable assemblies having twoacoustically isolated sections for a first implementation of adirectional sensitive fiber optic cable system, according to someimplementations.

FIG. 5 illustrates a third cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 6 illustrates a fourth cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 7 illustrates a fifth cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 8 illustrates a perspective view of a schematic of an exampledirectional sensitive fiber optic cable assembly for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 9 illustrates evanescent electromagnetic (EM) coupling in adouble-core optical fiber, according to some implementations.

FIG. 10 illustrates a cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly for a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 11 illustrates a first example of operation mode 1 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 12 illustrates a second example of operation mode 1 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 13 illustrates a first example of operation mode 2 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 14 illustrates a second example of operation mode 2 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 15 illustrates a first example of operation mode 3 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 16 illustrates a second example of operation mode 3 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations.

FIG. 17 illustrates a schematic of a fiber optic gyro, according to someimplementations.

FIG. 18 illustrates a cross section of a schematic of a directionalsensitive fiber optic cable assembly in a wellbore for isolating acompartment flow and a tubing flow, according to some implementations.

FIG. 19 illustrates a longitudinal view of a schematic of a directionalsensitive fiber optic cable assembly in a wellbore for isolating acompartment flow and a tubing flow, according to some implementation.

FIG. 20 illustrates a cross section of a schematic of a directionalsensitive fiber optic cable assembly in a wellbore for seismicacquisitions, according to some implementations.

FIG. 21 illustrates a longitudinal view of a schematic of a directionalsensitive fiber optic cable assembly in a wellbore for seismicacquisitions, according to some implementations.

FIG. 22 illustrates a flowchart of an example method for isolating acompartment flow and a tubing flow using a first implementation of adirectional sensitive fiber optic cable system, according to someimplementations.

FIG. 23 illustrates a flowchart of an example method for isolating acompartment flow and a tubing flow using a second implementation of adirectional sensitive fiber optic cable system, according to someimplementations.

DETAILED DESCRIPTION

Fiber optic cables with laser signals can be used for downhole sensingto collect data along a wellbore. For example, a distributed acousticsensing (DAS) system can use fiber optic cables connected to a laser boxto detect acoustic frequency strain signals in the wellbore. A lasersource or a laser transmitter in the laser box can send short laserpulses into a fiber. When light of the laser pulses travels towards theend of the fiber, the light interacts with crystal molecules inside thefiber and part of the light is scattered back to be detected by adetector or a receiver in the laser box. The detected light can beanalyzed to determine characteristics of the sound waves affecting thefiber.

However, fiber optic cables are, by nature, not able to sense adirection of an external force, because the fiber is affected by asurrounding environment without being able to detect the direction ofthe source. For instance, if a fiber optic cable of a DAS is affected bya sound wave from a seismic source, the DAS cannot determine whether thesound recorded is coming from above (for example, signals directly fromthe seismic source) or from a reflection point below or from the side(for example, signals reflected by earth subsurface layers below). Inother words, the fibers are affected by sources from every direction andnot directional sensitive.

This disclosure describes a directional sensitive fiber optic cablesystem for downhole sensing. In other words, the described fiber opticcable system enables directional sensitivity and can isolate externalforces (such as acoustic waves) or sense environmental variations (suchas pressure, strain-stress, or temperature changes) from differentdirections.

A first implementation of the fiber optic cable system is based on soundisolation. As illustrated later in FIGS. 1-8, the first implementationincludes a first fiber optic cable assembly including acoustic isolatingmaterial that acoustically isolates multiple fiber optic cables (orfibers) into different sections, each section facing a direction (forexample, east, south, west, north, up, down, left, or right). The fiberoptic cable(s) positioned in different sections can detect sound wavesreceived in individual sections so that sound waves coming fromdifferent directions associated with the sections can be isolated. Toimprove sound detectability and directional sensitivity, each sectionalso includes an acoustic reflective surface to amplify sound wavesreceived in the section. The acoustic reflective surface can also have acurved shape so that the sound waves are reflected towards the fiberoptic cable(s) in the section. Laser signals are sent to the multiplefiber optic cables, and the sound directions can be determined based onthe returned laser signals.

A second implementation of the fiber optic cable system is based onevanescent electromagnetic (EM) coupling. As illustrated later in FIGS.10-16, the second implementation includes a second fiber optic cableassembly including a signal fiber and multiple sensing fibers placedaround the signal fiber. Acoustic or pressure mirrors can be used toseparate the multiple sensing fibers into different sections facingdifferent directions. Laser signals are transmitted to the sensingfibers, where each sensing fiber carries a laser signal and each lasersignal has a different frequency than other laser signals. Environmentalvariations can change a refractive index of a sensing fiber, reduce adistance between the sensing fiber and the signal fiber, or both. Whenthe distance between the sensing fiber and the signal fiber is less thana threshold, and the sensing fiber and the signal fiber have similarrefractive indexes, evanescent coupling occurs where signal energytransfers from the sensing fiber to the signal fiber. The directions,amplitudes, or frequencies of the environmental variations or acousticwaves can be determined based on the signal frequencies and amplitudesof the evanescent coupling signals received in the signal fiber and thesignal intensity changes in the sensing fibers.

In some implementations, the described fiber optic cable assemblies canbe strapped outside a tubing (or a casing) and lowered into a wellborewith the tubing, where, for example, a first section is facing thetubing and a second section is facing away from the tubing. The fiber(s)in the first section can sense sound waves or environmental variationscaused by a tubing flow (that is, a fluid flowing through the tubing),and the detected signals from the first section can be used to determinecharacteristics of the tubing fluid. Similarly, the fiber(s) in thesecond section can sense sound waves or environmental variations causedby a compartment fluid (that is, a fluid flowing through an annulusbetween a formation and the tubing), and the detected signals from thesecond section can be used to determine characteristics of thecompartment fluid. In this disclosure, a fluid is flowing media, whichcan be a one-phase flow or a multiphase flow.

In some implementations, as described later in FIG. 17, the describedfiber optic cable system also includes a fiber optic gyro fordetermining an amount of tubing rotation that occurs when the tubing isrunning downhole. Based on the amount of tubing rotation, an orientationof the fiber optic cable assembly in the wellbore can be determined andthe direction of each section can also be determined. The fiber opticgyro can be wrapped around the tubing and lowered into the wellbore withthe tubing.

In some implementations, the described fiber optic cable system canconnect to one or more computers or processors to process receivedsignals from the fiber optic cable assembly or the fiber optic gyro orboth. The one or more computers or processors can also include acomputer-readable medium (for example, a non-transitorycomputer-readable medium) including instructions which, when executed,cause the one or more computers or processors to perform operations ofprocessing signals from the fiber optic cable assembly or the fiberoptic gyro or both as described in this disclosure.

In sum, the described fiber optic cable system can sense environmentalvariations from different directions. The described fiber optic cablesystem can be used in various scenarios. For example, as describedearlier and illustrated later in FIGS. 18-19, the fiber optic cablesystem can be used to separate a compartment flow and a tubing flow. Thefiber optic cable system can also be used for seismic applications toseparate down-going sound waves (for example, sound waves directly froma seismic source at an earth surface) and up-going sound waves (forexample, sound waves reflected by earth subsurface layers), asillustrated later in FIGS. 20-21.

The first implementation of the directional sensitive fiber optic cablesystem based on sound isolation

FIG. 1 illustrates a first cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly 100 for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations. The example fiber optic cableassembly 100 includes an elongate housing 102, multiple fiber opticcables (or fibers) 104 a-104 h placed inside the housing 102 andextending longitudinally along the housing, and acoustic isolatingmaterial 106 placed inside the housing 102 and extending longitudinallyalong the housing. The acoustic isolating material 106 is formed toinclude multiple outwardly radially extending arms 110 a-110 d extendingfrom a center of the housing 102 towards a circumference of the housing102. The multiple arms can divide a space inside the housing 102 (forexample, evenly or unevenly divide the space) into multiple acousticallyisolated sections (for example, four sections 114 a-114 d). The fiberoptic cable assembly 100 can have N acoustically isolated sections,where N is an integer number greater than one. Each acousticallyisolated section extends longitudinally, and includes at least one fiberoptic cable. Each acoustically isolated section is acousticallyinsulated from remaining sections of the multiple acoustically isolatedsections due to the arm separating two adjacent sections. Theacoustically isolated sections 114 a-114 d can have acoustic reflectingsurfaces 108 a-108 d.

For example, the acoustic isolating material 106 can be a star shape,filling the vertical crosshatching area in FIG. 1 and including fourarms 110 a-110 d to evenly divide the space inside the housing 102 intofour sections 114 a-114 d. Each section can have two fiber optic cables(for example, one cable for measuring temperature and one cable fordetecting sound waves). The fiber optic cables placed in differentsections are isolated from each other. For example, the fiber opticcables 104 a-104 b in section 114 a are isolated from the fiber opticcables 104 c-104 d in section 114 b. In some implementations, the fiberoptic cable assembly 100 can have less than or more than four arms, andeach section can have less than or more than two fiber optic cables.

For enhancing sound directional sensitivity, the acoustic reflectingsurfaces 108 a-108 d can use a hard (or high density) and acousticreflective material so that sound waves can be reflected. For example,the acoustic reflecting surfaces 108 a-108 d can be made of polyetherether ketone (PEEK) or other types of material. The acoustic reflectingsurfaces can be made by injection molding or other methods consistentwith this disclosure. To further enhance reflection and improve signaldirectional sensitivity, the acoustic reflecting surfaces 108 a-108 dcan have a shape that can reflect sound waves received in each sectiontowards the fiber optic cables within the section so that the fiberoptic cables can receive more sound energy. For example, the acousticreflecting surfaces 108 a-108 d can have a curved shape, such as aconical shape or a C-shape.

The acoustic isolating material 106 can reduce or prevent sound wavesreceived in one section from penetrating into another section. Whensound waves reach the acoustic reflecting surfaces 108 a-108 d, aportion of the waves undergoes reflection and a portion of the wavesundergoes transmission across the acoustic reflecting surfaces 108 a-108d. The sound wave that passes through the reflective surfaces 108 a-108d can be further reduced by the acoustic isolating material 106. Theacoustic isolating material 106 can be a soft material that can absorbsound. For example, the acoustic isolating material 106 can be acomposite material such as a mix of High Density Poly Ethylene (HDPE)with a Styrene Butadiene Rubber (SBR) or other types of materialconsistent with this disclosure. The materials of the acousticreflecting surfaces 108 a-108 d and the acoustic isolating material 106can be strong and light, and at the same time can survive and functionat temperatures up to, for example, 120-150° C. without breaking ormelting.

In some implementations, the housing 102 can have a circularcross-section, and be made of, for example, a metal, a compositematerial (with carbon fiber or PEEK), or other material that does notaffect sound penetrating from outside to inside of the housing 102. Thehousing 102 can be strong and light, and protect the fiber optic cables104 a-104 h from damaging and degradation. In some implementations, thefiber optic cables 104 a-104 h can be optical fibers without protectivecable tubes.

The fiber optic cable assembly 100 can also include a gel in eachacoustically isolated section to fill the void between the arms 110a-110 d, for example, filling the horizontal crosshatching areas inFIG. 1. The gels keep the fiber optic cables 104 a-104 h immobilized andprotect the fiber optic cables 104 a-104 h from external forces. Thegels can be a hydrophobic gel for preventing or reducing possiblehydrogen darkening or other types of gel consistent with thisdisclosure. In some implementations, the gels can be optional.

FIG. 2 illustrates a second cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly 200 for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations. The fiber optic cable assembly 200 issubstantially similar to (for example, the same as) the fiber opticcable assembly 100 in FIG. 1 except that the fiber optic cable assembly200 has additional isolating material 202 a-202 d that further protectsthe fiber optic cables 104 a-104 h beyond gels. For example, as shown inFIG. 2, the additional isolating material 202 a-202 d can fill thediagonal crosshatching areas around the fiber optic cables 104 a-104 h,and gels can fill the remaining horizontal crosshatching areas in theacoustically isolated sections.

FIG. 3 illustrates an example physical dimension of a schematic of adirectional sensitive fiber optic cable assembly 300 for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations. For example, the housing 102 can havea diameter of 0.5-1.5 inches. The center of the acoustic isolatingmaterial 106 can have a square shape with each side having a length of0.15-1.0 inches. The arms 110 a-110 d can have a thickness of 0.05-0.25inches. For example, since the acoustic reflecting surfaces 108 a-108 dcan have a curved shape, portions of the arms 110 a-110 d towards thecenter of the acoustic isolating material can have a thickness of 0.05inches, while portions of the arms 110 a-110 d towards the housing 102can be thicker and have a thickness of 0.25 inches. The dimensions areexamples only; other dimensions are possible and can depend on thespecific application for which the assembly 300 is being developed.

FIGS. 4A-4C illustrate cross sections of schematics of exampledirectional sensitive fiber optic cable assemblies 400 a, 400 b, and 400c having two acoustically isolated sections for a first implementationof a directional sensitive fiber optic cable system, according to someimplementations. The fiber optic cable assemblies 400 a, 400 b, and 400c can have three fiber optic cables in each acoustically isolatedsection. The acoustic reflecting surfaces 402, 404, 406, 408, 410, and412 have different curved shapes, where the acoustic reflecting surfaces402 and 404 in FIG. 4A are most curved and the acoustic reflectingsurfaces 410 and 412 in FIG. 4C are least curved. In some cases, thereflecting surfaces 402 and 404 can reflect more sound waves towards thefibers than the reflecting surfaces 410 and 412 can.

FIG. 5 illustrates a third cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly 500 for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations. The fiber optic cable assembly 500includes a housing 502, and acoustic isolating material 506 dividing aspace inside the housing 502 into four acoustically isolated sections.Each section has one fiber optic cable. In some implementations, eachsection can have more than one fiber optic cable. The four fiber opticcables 504 a-504 d are grooved in an inner surface of the housing 502through grooves 510 a-510 d. For example, the fiber optic cables 504a-504 d can be grooved at a center of the housing portion of eachsection. In some implementations, the housing 502 can have a thicknessthat can at least accommodate the grooves 510 a-510 d. The acousticreflecting surfaces 508 a-508 d of the four acoustically isolatedsections can have a parabola-shape such that, for example, reflectedsound waves 514 caused by incoming sound waves 512 can be focusedtowards the fiber optic cable 504 c to enhance signal directionalsensitivity. When incoming sound waves 512 hit the parabola-shapedsurface 508 c, the surface 508 c can reflect the sound waves 514 towardsa focal point where the fiber optic cable 504 c is located.

FIG. 6 illustrates a fourth cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly 600 for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations. The fiber optic cable assembly 600includes a housing 602, and acoustic isolating material 606 dividing aspace inside the housing 602 into four acoustically isolated sections.Each section has one fiber optic cable located at a center of thesection and a parabola-shaped acoustic reflecting surface. For example,when sound waves 610 reflect off the parabola-shaped surface 608, thesound waves 610 bounce out in straight lines, no matter where the soundwaves 610 hit the parabola-shaped surface 608, to a focal point wherethe fiber optic cable 604 locates.

FIG. 7 illustrates a fifth cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly 700 for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations. The fiber optic cable assembly 700includes a housing 702, and acoustic isolating material 706 dividing aspace inside the housing 702 into four acoustically isolated sections.Each section can have two fiber optic cables and an acoustic lens tofocus sound waves towards the fiber optic cables. For example, two fiberoptic cables 704 a-704 b and an acoustic lens 708 can be placed in theacoustically isolated section 714. The acoustic lens 708 can be locatedat a center of an inner surface of the housing portion associated withthe section 714. When incoming sound waves 710 pass through the lens708, the passed sound waves 712 are directed to where the fiber opticcables 704 a-704 b are located to enhance signal directionalsensitivity.

FIG. 8 illustrates a perspective view of a schematic of an exampledirectional sensitive fiber optic cable assembly 800 for a firstimplementation of a directional sensitive fiber optic cable system,according to some implementations. The fiber optic cable assembly 800has four acoustically isolated sections. Each section has two fiberoptic cables, for instance, one for measuring temperatures (DTSmeasurements) and one for sensing acoustic waves (DAS measurements). Forexample, fiber optic cables 802 a-802 d are used for acoustic sensing.In some implementations, one fiber optic cable can be used for both DTSand DAS measurements. A laser box 804 can connect to the fiber opticcables 802 a-802 d. The laser box 804 can include a transmitter 808 (ora laser source) and a receiver 810 (or a detector). The laser box 804can use a multiplexer (or a switch) 806 to multiplex the fiber opticcables 802 a-802 d. For example, the multiplexer 806 can connect thelaser box 804 to the fiber optic cables in an order of 802 a, 802 b, 802c, 802 d, back to 802 a, and so on. In some implementations, thetransmitter 808 can send a first laser pulse into a first fiber opticcable, wait for the receiver 810 to receive the returned laser pulse,then send a second laser pulse in a second fiber optic cable, and so on.The returned laser pulse can result from a reflection and scattering ofthe transmitted laser pulse. In some implementations, the transmitter808 can send, for instance, 10,000 pulses into a first fiber opticcable, wait for the receiver 810 to receive returned laser pulses, thensend another 10,000 pulses into a second fiber optic cable, and so on.In some implementations, the laser box 804 can connect to one or morecomputers or processors to configure pulse transmissions at thetransmitter 808, or process the returned pulses received at the receiver810, or both, using one or more software programs. As will be understoodby those of ordinary skill in the art, the laser box 804 can connect toany of the fiber optic cable assemblies in FIGS. 1-7.

The second implementation of the directional sensitive fiber optic cablesystem based on evanescent EM coupling

In some implementations, evanescent EM coupling can be used fordirectional sensitivity detection. FIG. 9 illustrates evanescent EMcoupling in a double-core optical fiber 900, according to someimplementations. The double-core optical fiber 900 includes a firstfiber core 902, a second fiber core 904, and claddings 906, 908, and910. The fiber cores 902 and 904 are separated by the cladding 906 witha distance d₀. Both fiber cores 902 and 904 have a refractive index n₁,and the claddings 906, 908, and 910 have a refractive index n₂.

Evanescent EM coupling occurs when the two fiber cores 902 and 904 arebrought sufficiently close (closer than a threshold as discussed later)and have similar refractive indexes. From a ray perspective, in anoptical fiber the core-cladding interface sets a condition for totalinternal reflection. If a laser signal or beam propagates at an angleequal to or greater than a threshold angle, the signal undergoes totalinternal reflection and becomes confined to propagate along the core ofthe fiber. Yet, due to wave nature of the electromagnetic radiation, asthe signal is completely reflected, some part of the signal or waveextends into the cladding and exponentially decays or evanesces. Theenergy flow of this evanescent signal or wave is parallel to the surfaceof the core and in a same direction as the main flow of energy withinthe core.

In other words, if the fiber cores 902 and 904 are close enough and havesimilar refractive indexes, when a laser signal is transmitted into onefiber core (also called input fiber core), an evanescent coupling signalor wave appears in the other fiber core (also called output fiber core).That is, the input fiber core can transfer signal energy to the outputfiber core through evanescent coupling. For example, when an excitationsignal of a power P₁(0) is sent to the first fiber core 902, if therefractive indexes n₁=n₂ and the distance

${d_{0} \leq \frac{1}{\beta_{1}}},$

where β₁ is the propagation constant in the first fiber core 902 asshown in Equation (1) later, then the signal powers in the fiber cores902 and 904, at any length z in a direction parallel to the wall of thefibers, can be expressed as

${{P_{1}(z)} = {{P_{1}(0)}\left( {1 - {\frac{\kappa^{2}}{\gamma^{2}}\sin^{2}\gamma \; z}} \right)}},{and}$${{P_{2}(z)} = {{P_{1}(0)}\left( {\frac{\kappa^{2}}{\gamma^{2}}\sin^{2}\gamma \; z} \right)}},$

where P₁(z) and P₂(z) represent the signal powers in the first fibercore 902 and the second fiber core 904, respectively, and P₂(z) is thepower of the evanescent coupling signal in the second fiber core 904responsive to the excitation signal in the first fiber core 902. Theevanescent coupling also changes the signal intensity in the first fibercore 902 from P₁(0) to P₁(z). In addition, K is a factor that depends onthe optical properties of the fiber, and y is defined as

${\gamma = \sqrt{\kappa^{2} + {\frac{1}{4}\left( {\beta_{1} - \beta_{2}} \right)^{2}}}},$

where β_(i) is the propagation constant in the i-th fiber core definedas

$\begin{matrix}{{\beta_{i} = {\frac{\omega^{2}}{c^{2}} = \frac{1}{\left( {2{\pi\lambda}_{i}} \right)^{2}}}},{i = 1},2} & (1)\end{matrix}$

where λ_(i) is the wavelength of the signal in the i-th fiber core. Forexample, λ₁ is the wavelength of the excitation signal in the firstfiber core 902.

FIG. 10 illustrates a cross section of a schematic of an exampledirectional sensitive fiber optic cable assembly 1000 for a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations. The example fiber optic cableassembly 1000 includes an elongate housing 1002, sensing fibers 1004a-1004 d, and a signal fiber 1006. The sensing fibers 1004 a-1004 d andthe signal fiber 1006 are placed inside the housing 1002 and extendedlongitudinally along the housing 1002. The housing 1002 can have acircular cross-section. The signal fiber 1006 can be placed at thecenter of the housing 1002, while the sensing fibers 1004 a-1004 d areplaced around the signal fiber 1006. For example, the sensing fibers1004 a-1004 d can be arranged equidistantly on a circle around thesignal fiber 1006. Although FIG. 10 shows four sensing fibers, as willbe understood by those of ordinary skill in the art, the fiber opticcable assembly 1000 can include N sensing fibers, where N>1.

The fiber optic cable assembly 1000 can also include acoustic mirrors1008 a-1008 d around the sensing fibers 1004 a-1004 d for focusing theincoming pressure or acoustic waves towards the sensing fibers 1004a-1004 d. The mirrors 1008 a-1008 d can have a parabolic shape, a conicshape, or other shapes. The mirrors 1008 a-1008 d can be made of densematerials and arranged to have a high impedance, for example, having animpedance higher than that of air or surrounding environments. In somecases, the mirrors 1008 a-1008 d can be made of metamaterial andarranged as an acoustic-photonic crystal. In some cases, the mirrors1008 a-1008 d can have a low mechanical coupling index so that themirrors 1008 a-1008 d reflect, instead of absorbing, the incomingpressure or acoustic waves. The mirrors 1008 a-1008 d can also have alow refractive index (for example, a refractive index close to therefractive index of air or surrounding environments or a refractiveindex close to one) to avoid disturbance to the coupling of theevanescent electromagnetic waves. The mirrors 1008 a-1008 d can divide aspace inside the housing 1002 into isolated sections 1010 a-1010 d,where each section extends longitudinally along the housing 1002, andeach section includes one of the sensing fibers 1004 a-1004 d. Eachsection corresponds to a direction. A high density fluid with a lowrefractive index (for example, a refractive index close to therefractive index of air or surrounding environments or a refractiveindex close to one) can fill the sections 1010 a-1010 d to keep thesensing fibers 1004 a-1004 d immobilized.

The sensing fibers 1004 a-1004 d (denoted as S_(i), i=1, . . . , 4) aresingle-mode fibers, each having a respective refractive index n_(i). Thesensing fibers 1004 a-1004 d can be connected to one or moretransmitters, and the one or more transmitters transmit laser signals tothe sensing fibers 1004 a-1004 d. Each sensing fiber S_(i) carries aninput laser signal of a distinct wavelength λ_(i), that is, λ₁≠λ₂≠λ₃≠λ₄.In other words, each sensing fiber carries a laser signal of a distinctfrequency (note that frequency and wavelength has a one-to-one mapping).The input laser signal can be a pulsed or continuous signal. The signalfiber 1006 (denoted as S₀) is a multi-mode fiber designed to carry lasersignals of the wavelength range in the sensing fibers 1004 a-1004 d. Inother words, the signal fiber 1006 can carry laser signals of one ormore of wavelengths λ₁, λ₂, λ₃, or λ₄. The signal fiber 1006 has arefractive index n₀, where n₀ can be constant or graded. The signalfiber 1006 can be connected to a receiver to receive evanescent couplingsignal(s) coupled from the input signals in the sensing fibers 1004a-1004 d. As discussed earlier, an evanescent coupling signal in thesignal fiber S₀ responsive to an input signal in the sensing fiber S_(i)has the same wavelength λ_(i). If each of the sensing fibers 1004 a-1004d causes an evanescent coupling signal in the signal fiber 1006, thesignal fiber 1006 can output evanescent coupling signals of wavelengthsλ_(i), λ₃, and λ₄. In some implementations, the fiber optic cableassembly 1000 can be connected to the laser box 804.

Assume that a distance from the sensing fiber S_(i) (i=1 N, N is thetotal number of the sensing fibers) to the signal fiber S₀ is d_(i),that the refractive index of the sensing fiber S_(i) is n_(i), and thatthe refractive index of the signal fiber S₀ is n₀. The distance d_(i)and the refractive index n_(i) of the sensing fiber can change withvariations in the environment around the fiber optic cable assembly1000, such as temperature, strain-stress, or pressure. For example, whena pressure or acoustic wave impacts on the fiber optic cable assembly1000, the distance d_(i) can change because the pressure or acousticwave can shift the sensing fiber S_(i). In some cases, the refractiveindex n_(i) can vary due to a change in temperature. When the changes inthe distance d_(i) and the refractive index n_(i) meet conditionsdiscussed earlier (that is, d_(i)≤1/β_(i), and n₀ and n_(i) have similarvalues), the sensing fiber S_(i) can transfer signal energy to thesignal fiber S₀.

As will be discuss in detail later, the following three modes ofoperation, can be used to couple signals from the sensing fibers to thesignal fiber:

-   -   (1) Evanescent coupling by distance shift: The fiber optic cable        assembly 1000 is designed such that d_(i)>1/β_(i) and n₀=n₁= . .        . =n_(N). Evanescent coupling occurs when the environmental        variation causes the distance shift such that d_(i)≤1/β_(i).    -   (2) Evanescent coupling by refractive index change: The fiber        optic cable assembly 1000 is designed such that d_(i)=1/β_(i)        and n_(i)≠n₀. Evanescent coupling occurs when the environmental        variation causes a change in the refractive index n_(i) such        that n_(i) and n₀ have similar values.    -   (3) Evanescent coupling by distance shift and refractive index        change: The fiber optic cable assembly 1000 is designed such        that d_(i)>1/β_(i) and n_(i)≠n₀. Evanescent coupling occurs when        the environmental variation causes d_(i)≤1/β_(i) and similar        values for n₀ and n_(i).

In operation modes 2 and 3, the refractive index of the sensing fiber istypically smaller than the refractive index of the signal fiber, thatis, n_(i)<n₀. This can be achieved by accurately doping the fiber corematerial. Furthermore, the refractive index of the shared claddingbetween the sensing fiber and the signal fiber is significantly smallerthan the refractive index of the sensing fiber, that is,n_(cladding)<min(n₁, . . . n_(N)).

In some implementations, by analyzing frequency component(s) andamplitude(s) of the coupled evanescent signal(s) in the signal fiber,the orientation and strength of the environmental perturbation can bedetermined. In some cases, the amplitude of an evanescent couplingsignal can depend on the separation distance between the coupled fibers.For example, a stronger environmental perturbation can shift the sensingfiber more, causing a smaller separation distance that leads to astronger evanescent coupling signal. In addition, as discussed earlier,each sensing fiber carries an input signal of a distinct wavelength) (orfrequency). For example, if the evanescent coupling signal received fromthe signal fiber includes wavelengths λ₁ and λ₂, then the environmentalperturbation is determined to come from the directions corresponding tothe sensing fibers S₁ and S₂. Based on the amplitude of the evanescentcoupling signal of wavelength λ₁, the strength of the environmentalperturbation from the direction of the sensing fiber S₁ can bedetermined. Similarly, based on the amplitude of the evanescent couplingsignal of wavelength λ₂, the strength of the environmental perturbationfrom the direction of the sensing fiber S₂ can be determined.

In some implementations, each sensing fiber can also act as a regulardistributed fiber optic sensor. As a result, in addition to the signalfiber connected to the receiver for receiving the evanescent couplingsignals, the sensing fibers can also be connected to one or morereceivers for receiving the reflected signals in the sensing fibers. Byanalyzing the reflected signals in the sensing fibers and the coupledevanescent signals in the signal fiber, the direction, amplitude, andfrequency of the environmental disturbance or pressure/acoustic wave canbe determined. For example, the reflected signal within the sensingfibers due to an environmental disturbance can be analyzed (forinstance, analyzing the signal intensity changes in the sensing fibers)using known distributed fiber optics techniques. The results can becorrelated and used with the data in the signal fiber to improve theaccuracy of the directional sensing.

Operation Mode 1: Evanescent Coupling by Distance Shift

In operation mode 1, the sensing fiber S_(i) is located at a distanced_(i)>1/β_(i) measured from the perimeter of the signal fiber S₀, whereβ_(i) is determined by the wavelength of the input signal in the sensingfiber S_(i) as shown in Equation (1). The signal fiber and the sensingfibers are assumed to have the same refractive index, that is, n₀=n₁= .. . =n_(N), where N is the total number of sensing fibers. The signalfiber is anchored or fixed to center of the fiber bundle, and thesensing fibers are displaced along the radial direction of the fiberbundle and not anchored. As a result, the fiber optic cable assembly1000 is set in a state of unstable equilibrium so that any environmentalperturbation can displace one, or multiple, sensing fibers along theradial direction. Evanescent coupling between any sensing fibers S_(i)and the signal fiber S₀ occurs if, and only if, d_(i)≤1/β_(i). In otherwords, if the displacement Δd_(i) caused by the environmentalperturbation is such that d_(i)−Δd_(i)≤1/β_(i), then the signal can becoupled from the corresponding sensing fiber S_(i) to the signal fiberS₀.

FIG. 11 illustrates a first example 1100 of operation mode 1 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations. The example 1100 includes a signalfiber 1102, and two sensing fibers 1104 and 1106 (that is, S₁ and S₂).Input signals of different wavelengths λ₁ and λ₂ are transmitted to thesensing fibers 1104 and 1106, respectively. The two sensing fibers 1104and 1106 are initially located at distances d₁ and d₂ from the signalfiber, respectively, where d₁>1/β₁ and d₂>1/β₂. In some implementations,d₁>

${{\max \left( {\frac{1}{\beta_{1}},\frac{1}{\beta_{2}}} \right)}\mspace{14mu} {and}\mspace{14mu} d_{2}} > {{\max \left( {\frac{1}{\beta_{1}},\frac{1}{\beta_{2}}} \right)}.}$

An incoming perturbation 1108 displaces the sensing fiber 1106 and movesthe sensing fiber 1106 closer to the signal fiber 1102. When theseparation distance between the sensing fiber 1106 and the signal fiber1102 is smaller than or equal to the separation threshold 1/β₂, thenevanescent coupling takes place and a portion of the signal ofwavelength λ₂ is transferred from the sensing fiber 1106 to the signalfiber 1102.

FIG. 12 illustrates a second example 1200 of operation mode 1 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations. Same as the example 1100, the example1200 includes the signal fiber 1102 and the two sensing fibers 1104 and1106. In addition to the incoming perturbation 1108 that moves thesensing fiber 1106 closer to the signal fiber 1102, the example 1200includes another incoming perturbation 1202 that moves the sensing fiber1104 closer to the signal fiber 1102. When the separation distancebetween the sensing fiber 1104 and the signal fiber 1102 is smaller thanor equal to the separation threshold 1/β₁, and the separation distancebetween the sensing fiber 1106 and the signal fiber 1102 is smaller thanor equal to the separation threshold 1/β₂, then evanescent couplingtakes place, where a portion of the signal of wavelength λ₁ istransferred from the sensing fiber 1104 to the signal fiber 1102, and aportion of the signal of wavelength λ₂ is transferred from the sensingfiber 1106 to the signal fiber 1102.

Operation Mode 2: Evanescent Coupling by Refractive Index Change

In operation mode 2, the sensing fiber S_(i) is located at a distanced_(i)=1/β_(i) measured from the perimeter of the signal fiber S₀. Thesensing fibers have different refractive indexes than the signal fiber,that is, n_(i)≠n₀ and typically n_(i)<n₀ for i=1, . . . N. The signaland sensing fibers are anchored to prevent any radial displacement. As aresult, the fiber optic cable assembly 1000 is set in a state of stableequilibrium. Evanescent coupling between any sensing fiber S_(i) and thesignal fiber S₀ occurs if, and only if, n_(i)=n₀. When the refractiveindex change Δn_(i) due to environmental variations is such thatn_(i)+Δn_(i)=n₀, then the signal can couple from the correspondingsensing fiber S_(i) to the signal fiber S₀. The refractive index change,Δn_(i), can be induced by any environmental factor, such as temperature,pressure, and strain.

FIG. 13 illustrates a first example 1300 of operation mode 2 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations. The example 1300 includes a signalfiber 1302, and two sensing fibers 1304 and 1306 (that is, S₁ and S₂).Input signals of different wavelengths λ₁ and λ₂ are transmitted intothe sensing fibers 1304 and 1306, respectively. The two sensing fibers1304 and 1306 are located at distances d₁ and d₂ from the signal fiber,respectively, where d₁=1/β_(i) and d₂=1/β₂. The two sensing fibers 1304and 1306 have the refractive index n₁ and n₂, respectively, where n₁≠n₀,and n₂≠n₀. An incoming perturbation 1308 changes the refractive index ofthe sensing fiber 1306 such that n₂+Δn₂=n₀. As a result, evanescentcoupling takes place and a portion of the signal of wavelength λ₂ istransferred from the sensing fiber 1306 to the signal fiber 1302.

FIG. 14 illustrates a second example 1400 of operation mode 2 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations. Same as the example 1300, the example1400 includes the signal fiber 1302 and the two sensing fibers 1304 and1306. In addition to the incoming perturbation 1308 that increases therefractive index of the sensing fiber 1306 to n₀, the example 1400includes another incoming perturbation 1402 that increases therefractive index of the sensing fiber 1304 to n₀. As a result,evanescent coupling takes place, where a portion of the signal ofwavelength λ₁ is transferred from the sensing fiber 1304 to the signalfiber 1302, and a portion of the signal of wavelength λ₂ is transferredfrom the sensing fiber 1306 to the signal fiber 1302.

Operation Mode 3: Evanescent Coupling by Refractive Index Change andDisplacement

Operation mode 3 is a combination of operation modes 1 and 2. Thesensing fiber S_(i) is located at a distance d_(i)>1/β_(i) measured fromthe perimeter of the signal fiber S₀. In addition, the refractive indexof the sensing fiber S_(i) is set so that n_(i)≠n₀. In operation mode 3,the signal fiber is anchored to the center of fiber bundle, and thesensing fibers can be displaced along the radial direction of the fiberbundle and not anchored. As a result, the fiber optic cable assembly1000 is set in a state of unstable equilibrium so that any perturbationcan displace, along the radial direction, and change the refractiveindex of one, or multiple, sensing fibers. Evanescent coupling betweenany sensing fiber S_(i) and the signal fiber S₀ occurs if, and only if,d_(i)≤1/β_(i) and n_(i)=n₀. If these conditions are met, signal energycan be transferred from the corresponding sensing fiber S_(i) to thesignal fiber S₀.

FIG. 15 illustrates a first example 1500 of operation mode 3 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations. The example 1500 includes a signalfiber 1502, and two sensing fibers 1504 and 1506. Input signals ofdifferent wavelengths λ₁ and λ₂ are transmitted to the sensing fibers1504 and 1506, respectively. The two sensing fibers 1504 and 1506 arelocated at distances d₁ and d₂ from the signal fiber, respectively,where d₁>1/β₁ and d₂>1/β₂. A perturbation 1508 displaces the sensingfiber 1506 and moves the sensing fiber 1506 closer to the signal fiber1502. In addition, a perturbation 1510 changes the refractive index ofthe sensing fiber 1506 to n₀. When the separation distance between thesensing fiber 1506 and the signal fiber 1502 is smaller than or equal tothe separation threshold 1/β₂ and the refractive index of the sensingfiber 1506 becomes n₀, then evanescent coupling takes place and aportion of the signal of wavelength λ₂ is transferred from the sensingfiber 1506 to the signal fiber 1502.

FIG. 16 illustrates a second example 1600 of operation mode 3 for adirectional sensitive fiber optic cable assembly of a secondimplementation of a directional sensitive fiber optic cable system,according to some implementations. Same as the example 1500, the example1600 includes the signal fiber 1502 and the two sensing fibers 1504 and1506. In addition to the perturbation 1508 that reduces the distancebetween the sensing fiber 1506 and the signal fiber 1502 to be smallerthan or equal to 1/β₂ and the perturbation 1510 that changes therefractive index of the sensing fiber 1506 to n₀, the example 1600includes a perturbation 1602 that reduces the distance between thesensing fiber 1504 and the signal fiber 1502 to be small than or equalto 1/β_(i), and a perturbation 1604 that changes the refractive index ofthe sensing fiber 1504 to n₀. As a result, evanescent coupling takesplace, where a portion of the signal of wavelength λ₁ is transferredfrom the sensing fiber 1504 to the signal fiber 1502, and a portion ofthe signal of wavelength λ₂ is transferred from the sensing fiber 1506to the signal fiber 1502.

The directional sensitive fiber optic cable assembly in FIG. 1-7 or 10can be strapped outside a tubing (or a casing), as will be shown inFIGS. 18-21, and lowered into a wellbore with the tubing. The laser box804 can be at the terranean surface of the wellbore. For example, clampscan be used to strap the fiber optic cable assembly as a straight linelongitudinally along the tubing. In some implementations, thedirectional sensitive fiber optic cable assembly can be marked on oneside with a scribe line indicating a direction, for example, a directionof up or out. In some cases, the scribe line can be longitudinallymarked along a center of the housing portion of an isolated section. Thepurpose of the marking is to make sure that the same isolated section isfacing in the same direction along the tubing, for example, outwardsfrom the tubing. During installation, the fiber optic cable assembly canbe strapped outside the tubing with the scribe line facing up or outwith respect to the tubing so that one isolated section is facing awayfrom the tubing and another section is facing the tubing.

After the fiber optic cable assembly is strapped outside the tubing, thefiber optic cable assembly will be run into the wellbore together withthe tubing, where the tubing rotates slowly in one direction whenrunning into the wellbore. The rotation stops once the tubing is settledin the wellbore. To understand the orientation of the fiber optic cableassembly in the wellbore, a fiber optic gyro can be used to estimate theamount of the rotation that occurred when the tubing is running into thewellbore. For example, for a horizontal wellbore, the orientation of thefiber optic cable assembly in the wellbore can provide information onwhich isolated section is facing up towards the earth surface (or at ahigh side) and which isolated section is facing down away from the earthsurface (or at a low side).

FIG. 17 illustrates a schematic of a fiber optic gyro 1700, according tosome implementations. The fiber optic gyro 1700 can be used to determinean amount of tubing rotation that occurs when the tubing is runningdownhole. The fiber optic gyro 1700 includes two optical fibers 1702 and1704 (illustrated as the dash line and the solid line, respectively)wrapped on an outer surface of the tubing 1706 and wrapped in oppositedirections from each other. The optical fibers 1702 and 1704 can bewrapped at an end of the tubing 1706 towards downhole so that a fullrotation can be estimated when the tubing 1706 running from the surfaceof the wellbore to the downhole. For example, the fibers 1702 and 1704can wrap around the tubing 1706 and run into the wellbore with thetubing 1706. In some implementations, the fibers 1702 and 1704 can wrap,for example, 20-40 turns around the tubing 1706. The fibers 1702 and1704 are different than the fibers for directional sensitivitydetection, for example, 104 a-104 h, 1004 a-1004 d, and 1006. In someimplementations, the two optical fibers 1702 and 1704 can be in onefiber optic cable where the two fibers are in one protective cable tube.

The fibers 1702 and 1704 can connect to a laser box 1712 at theterranean surface of the wellbore. The laser box 1712 can include atransmitter (or a laser source) 1708 and a receiver (or a detector)1710. The transmitter 1708 can connect to one end of the fiber 1702 andthe receiver 1710 can connect to the other end of the fiber 1702.Similarly, the transmitter 1708 can connect to one end of the fiber 1704and the receiver 1710 can connect to the other end of the fiber 1704.The transmitter 1708 can emit laser lights or signals with a specificfrequency and wavelength into the fibers 1702 and 1704. Because thefibers 1702 and 1704 are wrapped in opposite directions, the laserlights emitted to the fibers 1702 and 1704 are travelled in oppositedirections around the tubing 1706. The receiver 1710 can receivereturned laser lights from the fibers 1702 and 1704, and determine anamount of the tubing rotation based on the received laser lights. Insome implementations, the laser box 1712 can connect to one or morecomputers or processors to configure pulse transmissions at thetransmitter 1708, or process the returned laser lights receiving at thereceiver 1710 to determine the tubing rotation, or both, using one ormore software programs.

The fiber optic gyro 1700 can estimate an angular velocity of the tubingrotation based on a Sagnac effect, and further estimate the amount ofthe tubing rotation based on the angular velocity. For example, thetransmitter 1708 emits a first laser light into the fiber 1702 and asecond laser light into the fiber 1704, and the receiver 1710 receivesthe returned first laser light and the returned second laser light. Thefirst laser light and the second laser light can be transmitted at thesubstantially similar times (for example, at the same time) or differenttimes. Based on the received laser lights, the computer or the processorconnected to the receiver 1710 can determine a first time duration, t₁,for the first laser light to travel through the fiber 1702 and a secondtime duration, t₂, for the second laser light to travel through thefiber 1704. The angular velocity of the tubing rotation can bedetermined based on a difference between t₁ and t₂, Δt, by solving ω inthe following equation:

${{\Delta \; t} = {{t_{1} - t_{2}} = \frac{4\pi \; R^{2}\omega}{c^{2} - {R^{2}\omega^{2}}}}},$

where ω is the angular velocity of the tubing rotation (for example, ina unit of radians per second), R is the radius of the tubing, and c isthe speed of light. In some implementations, a time duration for thetubing running from the surface to the downhole, T, can be determined,and the amount of tubing rotation (for example, in a unit of radians)can be determined by ω*T. The orientation of the directional sensitivefiber optic cable assembly can be determined based on the amount oftubing rotation, for example, by determining a number of full turns thetubing has made and the angle of the partial turn.

In some implementations, the laser box 804 in FIG. 8 and the laser box1712 in FIG. 17 can be the same or different laser boxes. In case of thesame laser box, when the tubing is running into the wellbore, the laserbox can first connect to the fiber optic gyro 1700 to transmit andreceive laser signals. After the tubing has stopped the rotation andsettled into the wellbore, the laser box can then connect to the fiberoptic cable assembly in FIG. 1-7 or 10.

The directional sensitive fiber optic cable assembly can be used fordifferent applications, such as determining directions of seismic soundwaves, separating a compartment flow and a tubing flow, determining atype of fluid flowing within a compartment, cross flow detection, andother scenarios and use cases.

FIG. 18 illustrates a cross section 1800 of a schematic of a directionalsensitive fiber optic cable assembly in a wellbore for isolating acompartment flow and a tubing flow, according to some implementations.The cross section 1800 includes a directional sensitive fiber opticcable assembly 1802 strapped outside a tubing 1804 in a wellbore 1806.The fiber optic cable assembly 1802 can be an assembly in FIG. 1-7 or10. Fluids 1810 (that is, inflow from reservoir), for example,hydrocarbon fluids, can flow from a formation (or a reservoir) innersurface 1808 into a compartment 1812 (that is, an annulus between theformation inner surface 1808 and the tubing 1804) and the wellbore 1806.In some implementations, with inflow control devices (ICDs) or intervalcontrol valves (ICVs) on the tubing 1804, fluids 1810 can flow into thewellbore 1806.

FIG. 19 illustrates a longitudinal view 1900 of a schematic of adirectional sensitive fiber optic cable assembly in a wellbore forisolating a compartment flow and a tubing flow, according to someimplementation. The longitudinal view 1900 includes a fiber optic cableassembly 1906 strapped outside a tubing 1902 in a wellbore 1904, and aformation inner surface 1908. The fiber optic cable assembly 1906 can bean assembly in FIG. 1-7 or 10. The wellbore 1904 can be a horizontal orvertical wellbore. Fluids 1918 (that is, inflow from reservoir), such ashydrocarbon fluids, can flow from the formation (or a reservoir) innersurface 1908 into a compartment 1916 (that is, an annulus between theformation inner surface 1908 and the tubing 1902), and further flow intothe wellbore 1904 through ICDs or ICVs 1912 on the tubing 1902. Thedown-flowing fluid (flowing from uphole to downhole) in the compartment1916 is a compartment flow 1910, and the up-flowing fluid (flowing fromdownhole to uphole) in the tubing 1902 (or in the wellbore 1904) is atubing flow 1914. The longitudinal view 1900 also includes packers 1920.

The fiber optic cable assembly 1906 can be used to differentiate betweenthe down-flowing compartment flow 1910 and the up-flowing tubing flow1914. For example, when strapping the fiber optic cable assembly 1906outside the tubing 1902, based on the scribe line marked on the fiberoptic cable assembly 1906, a first isolated section can face the tubing1902 to sense the sound or environmental variation caused by the tubingflow 1914, and a second isolated section can face the compartment 1916(or face away from the tubing 1902) to sense the sound or environmentalvariation caused by the compartment flow 1910.

For example, for the first implementation of the directional sensitivefiber optic cable system in FIGS. 1-7, when the transmitter 808 sendslaser pulses into the fiber optic cables of the fiber optic cableassembly 1906, a computer or processor connected to the receiver 810 canidentify received laser pulses from the first and second acousticallyisolated sections. Based on the received laser pulses from the first andsecond acoustically isolated sections, the computer or the processor candetermine flow velocities for the compartment flow 1910 and the tubingflow 1914, and further determine fluid densities and fluid compositionsfor the compartment flow 1910 and the tubing flow 1914. In someimplementation, the fiber optic gyro is used to determine orientation ofeach acoustically isolated section in the wellbore.

In some implementations, the computer or the processor can determineeddy currents along the fiber optic cables in the first and secondacoustically isolated sections. The eddy currents represent smallvariations in a pressure sound level. From the eddy currents, awavenumber-frequency plot can be generated. Based on thewavenumber-frequency plot, the computer or the processor can determineflow velocities of the compartment flow 1910 and the tubing flow 1914using Doppler shift effects generated by the sound waves of thecompartment flow 1910 and the tubing flow 1914 (for example, a greaterfrequency for the down-flowing fluid of the compartment flow 1910 and alesser frequency for the up-flowing fluid of the tubing flow 1914).Based on the flow velocities, fluid densities of the compartment flow1910 and the tubing flow 1914 can be estimated. Based on the fluiddensities, fluid compositions of the compartment flow 1910 and thetubing flow 1914 can be determined. In some implementations, using arrayprocessing, speeds of sound, not just flow velocities, of thecompartment flow 1910 and the tubing flow 1914 can be estimated. Basedon the speeds of sound, fluid compositions of the compartment flow 1910and the tubing flow 1914 can be determined, for example, the speed ofsound in gas is different from that in oil or water.

For the second implementation of the directional sensitive fiber opticcable system in FIG. 10, the transmitter 808 can transmit laser signalsof different wavelengths λ₁ and λ₂ to the sensing fibers in the firstsection facing the tubing 1902 and the second section facing thecompartment 1916, respectively. By analyzing the amplitude of theevanescent coupling signal of wavelength λ₁ in the signal fiber, as wellas the signal intensity change in the sensing fiber in the firstsection, the pressure sound level of the tubing flow 1914 can bedetermined. Based on the pressure sound level, the flow velocity of thetubing flow 1914 can be determined. Similarly, the amplitude of theevanescent coupling signal of wavelength λ₂ in the signal fiber, as wellas the signal intensity change in the sensing fiber in the secondsection, the flow velocity of the compartment flow 1910 can bedetermined.

FIG. 20 illustrates a cross section 2000 of a schematic of a directionalsensitive fiber optic cable assembly in a wellbore for seismicacquisitions, according to some implementations. The cross section 2000includes a directional sensitive fiber optic cable assembly 1802strapped outside a tubing 1804 in a wellbore 1806. The wellbore 1806 canbe a horizontal wellbore. Fluids 1810, for example, hydrocarbon fluids,can flow from a formation (or a reservoir) inner surface 1808 into thewellbore 1806. The fiber optic cable assembly 1802 can be used to sensedown-going sound 2002 caused by seismic signals directly from a seismicsource at an earth surface and up-going sound 2004 caused by seismicsignals reflected by earth subsurface layers below the tubing 1804.

FIG. 21 illustrates a longitudinal view 2100 of a schematic of adirectional sensitive fiber optic cable assembly in a wellbore forseismic acquisitions, according to some implementations. Thelongitudinal view 2100 includes a fiber optic cable assembly 1906strapped outside a tubing 1902 in a wellbore 1904, and a formation innersurface 1908.

The fiber optic cable assembly 1906 can be used to isolate down-goingseismic signals 2102 directly from a seismic source at an earth surfaceand up-going seismic signals 2104 reflected by earth subsurface layersbelow the tubing 1902, and enable four-dimensional seismic acquisitions.For example, the fiber optic gyro can be used to determine orientationof each isolated section of the fiber optic cable assembly 1906 in thewellbore. A computer or processor connected to the fiber optic gyro candetermine a first isolated section that is facing up towards the earthsurface and a second isolated section that is facing down away from theearth surface.

For the first implementation of the directional sensitive fiber opticcable system in FIGS. 1-7, the transmitter 808 can send laser pulsesinto the fiber optic cables of the fiber optic cable assembly 1906, anda computer or processor connected to the receiver 810 can identifyreceived laser pulses from the first and second isolated sections. Basedon the received laser pulses from the first and second isolatedsections, the computer or the processor can characterize, for example,the pressure sound levels of the down-going seismic signals 2102 and theup-going seismic signals 2104, and further determine, for example, fluidcompositions in earth subsurface layers.

For the second implementation of the directional sensitive fiber opticcable system in FIG. 10, the transmitter 808 can transmit laser signalsof different wavelengths λ_(i) and λ₂ to the sensing fibers in the firstsection facing the down-going seismic signals 2102 and the secondsection facing the up-going seismic signals 2104, respectively. Byanalyzing the amplitudes of the evanescent coupling signals ofwavelengths λ₁ and λ₂ in the signal fiber, as well as the signalintensity changes in the sensing fibers in the first and secondsections, the pressure sound levels of the down-going seismic signals2102 and the up-going seismic signals 2104 can be determined.

FIG. 22 illustrates a flowchart of an example method 2200 for isolatinga compartment flow and a tubing flow using a first implementation of adirectional sensitive fiber optic cable system, according to someimplementations. The method 2200 can be used for the flow isolatingdescribed in FIGS. 18-19. At block 2202, a fiber optic cable assembly inFIGS. 1-7 is strapped outside a tubing as shown in FIGS. 18-19. Forexample, the fiber optic cable assembly is strapped so that oneacoustically isolated section is facing away from the tubing formonitoring the compartment flow and another acoustically isolatedsection is facing the tubing for monitoring the tubing flow. A fiberoptic gyro is also wrapped around the outer surface of the tubing asshown in FIG. 17. The fiber optic cable assembly and the fiber opticgyro are lowered into the wellbore together with the tubing. At block2204, the fiber optic gyro is connected to the laser box 1712. Asdescribed in FIG. 17, based on the transmitted and returned laserpulses, the fiber optic gyro determines an amount of tubing rotationthat occurs when the tubing is running downhole, and therefore determinean orientation of the fiber optic cable system when the fiber opticcable system is settled in the wellbore.

At block 2206, the fiber optic cable assembly is connected to the laserbox 804. As described in FIG. 8, the transmitter 808 transmits laserpulses to the fibers positioned in the acoustically isolated sections ofthe fiber optic cable assembly. At block 2208, the receiver 810 receivesreturned laser pulses from the fibers in the fiber optic cable assembly.The laser box 804 and 1712 can be the same laser box, where the singlelaser box can first connect to the fiber optic gyro when the tubing isrunning downhole. Once the tubing is settled in the wellbore, the laserbox is then switched to connect to the fiber optic cable assembly. Atblock 2210, as discussed earlier, based on the received laser pulsesfrom the section facing the tubing, the flow velocity of the tubing flowis determined based on the pressure sound level. Similarly, based on thereceived laser pulses from the section facing away the tubing, the flowvelocity of the compartment flow is determined.

FIG. 23 illustrates a flowchart of an example method 2300 for isolatinga compartment flow and a tubing flow using a second implementation of adirectional sensitive fiber optic cable system, according to someimplementations. Similar to the method 2200, the method 2300 can be usedfor the flow isolating described in FIGS. 18-19. At block 2302, a fiberoptic cable assembly in FIG. 10 is strapped outside a tubing as shown inFIGS. 18-19. For example, the fiber optic cable assembly is strapped sothat one isolated section is facing away from the tubing for monitoringthe compartment flow and another isolated section is facing the tubingfor monitoring the tubing flow. A fiber optic gyro is also wrappedaround the outer surface of the tubing as shown in FIG. 17. The fiberoptic cable assembly and the fiber optic gyro are lowered into thewellbore together with the tubing. At block 2304, the fiber optic gyrois connected to the laser box 1712. As described in FIG. 17, based onthe transmitted and returned laser pulses, the fiber optic gyrodetermines an amount of tubing rotation that occurs when the tubing isrunning downhole, and therefore determine an orientation of the fiberoptic cable system when the fiber optic cable system is settled in thewellbore.

At block 2306, the fiber optic cable assembly is connected to the laserbox 804. As described in FIG. 8, the transmitter 808 transmits lasersignals of different wavelengths to the sensing fibers in the fiberoptic cable assembly. For example, laser signals of wavelengths λ₁ andλ₂ can be transmitted to the sensing fibers in the sections facing thetubing and facing the compartment, respectively. At block 2308, thereceiver 810 receives evanescent coupling signals from the signal fiberin the fiber optic cable assembly. The laser box 804 and 1712 can be thesame laser box, where the single laser box can first connect to thefiber optic gyro when the tubing is running downhole. Once the tubing issettled in the wellbore, the laser box is then switched to connect tothe sensing fibers and the signal fiber. At block 2310, flow velocitiesof the tubing flow and the compartment flow are determined based on thereceived evanescent coupling signals. For example, as discussed earlier,based on the amplitude of the evanescent coupling signal of wavelengthλ₁, the flow velocity of the tubing flow is determined based on thepressure sound level. Similarly, based on the amplitude of theevanescent coupling signal of wavelength λ₂, the flow velocity of thecompartment flow is determined.

Described implementations of the subject matter can include one or morefeatures, alone or in combination.

For example, in a first implementation, a fiber optic cable assembly,comprising: an elongate housing; a signal fiber placed inside thehousing and extending longitudinally; and a plurality of sensing fibersplaced inside the housing and extending longitudinally, wherein theplurality of sensing fibers are placed around the signal fiber, each ofthe plurality of sensing fibers carries a respective laser signal of adistinct frequency, and the signal fiber carries one or more evanescentcoupling signals responsive to the laser signals in the plurality ofsensing fibers.

The foregoing and other described implementations can each, optionally,include one or more of the following features:

A first feature, combinable with any of the following features, whereinthe plurality of sensing fibers placed around the signal fiber includesthe plurality of sensing fibers arranged in a circle with the signalfiber placed in the middle of the housing.

A second feature, combinable with any of the previous or followingfeatures, further comprising a high density fluid to keep the signalfiber and the plurality of sensing fibers in the housing immobilized.

A third feature, combinable with any of the previous or followingfeatures, further comprising: a plurality of mirrors dividing a spaceinside the housing into a plurality of isolated sections, each isolatedsection extending longitudinally, each isolated section including one ofthe plurality of sensing fibers.

A fourth feature, combinable with any of the previous or followingfeatures, wherein the housing has a circular cross-section.

A fifth feature, combinable with any of the previous or followingfeatures, further comprising: a transmitter transmitting the respectivelaser signals of the distinct frequencies into the plurality of sensingfibers; and a receiver receiving the one or more evanescent couplingsignals in the signal fiber.

A sixth feature, combinable with any of the previous or followingfeatures, wherein a distance between the signal fiber and one of theplurality of sensing fibers changes with an environmental variation, andthe change in the distance causes an evanescent coupling signal in thesignal fiber corresponding to the laser signal in the one of theplurality of sensing fibers, and the evanescent coupling signal has asame frequency as the laser signal in the one of the plurality ofsensing fibers.

A seventh feature, combinable with any of the previous or followingfeatures, wherein the environmental variation includes at least one oftemperature, strain-stress, or pressure change around the fiber opticcable assembly.

An eighth feature, combinable with any of the previous or followingfeatures, wherein frequencies of the one or more evanescent couplingsignals in the signal fiber are used to determine one or more directionsassociated with the environmental variation.

A ninth feature, combinable with any of the previous or followingfeatures, wherein a refraction index of one of the plurality of thesensing fibers changes with an environmental variation, and the changein the refraction index causes an evanescent coupling signal in thesignal fiber corresponding to the laser signal in the one of theplurality of sensing fibers, and the evanescent coupling signal has asame frequency as the laser signal in the one of the plurality ofsensing fibers.

A tenth feature, combinable with any of the previous or followingfeatures, further comprising a strap to secure the fiber optic cableassembly outside a tubing in a wellbore formed in a formation.

An eleventh feature, combinable with any of the previous or followingfeatures, further comprising: a fiber optic gyro including a first fiberand a second fiber wrapped on an outer surface of the tubing, the firstfiber and the second fiber wrapped in opposite directions from eachother, wherein the fiber optic gyro is used to determine an orientationof the fiber optic cable assembly after the tubing is settled in thewellbore.

In a second implementation, a method, comprising: transmitting lasersignals to a plurality of sensing fibers in a fiber optic cableassembly, wherein each sensing fiber carries one laser signal, eachlaser signal has a distinct frequency, and the fiber optic cableassembly comprises: an elongate housing; and a signal fiber and theplurality of sensing fibers placed inside the housing and extendinglongitudinally, wherein the plurality of sensing fibers are placedaround the signal fiber, the signal fiber carries one or more evanescentcoupling signals responsive to the laser signals in the plurality ofsensing fibers, and the fiber optic cable assembly is strapped outside atubing in a wellbore formed in a formation; receiving, from the signalfiber of the fiber optic cable assembly, the one or more evanescentcoupling signals; and based on the one or more evanescent couplingsignals, determining flow velocities of first flowing media flowingthrough the tubing and second flowing media flowing through an annulusbetween the formation and the tubing.

The foregoing and other described implementations can each, optionally,include one or more of the following features:

A first feature, combinable with any of the following features, whereinthe wellbore is a horizontal wellbore, and the method further comprisingusing a fiber optic gyro to determine an orientation of the fiber opticcable assembly in the wellbore.

A second feature, combinable with any of the previous or followingfeatures, wherein determining the flow velocities includes determiningpressure levels of the first flowing media and the second flowing mediabased on amplitudes of the one or more evanescent coupling signals.

A third feature, combinable with any of the previous or followingfeatures, wherein a distance between the signal fiber and one of theplurality of sensing fibers changes with an environmental variation, andthe change in the distance causes an evanescent coupling signal in thesignal fiber corresponding to the laser signal in the one of theplurality of sensing fibers, and the evanescent coupling signal has asame frequency as the laser signal in the one of the plurality ofsensing fibers.

A fourth feature, combinable with any of the previous or followingfeatures, wherein the environmental variation includes at least one oftemperature, strain-stress, or pressure change around the fiber opticcable assembly.

A fifth feature, combinable with any of the previous or followingfeatures, wherein a refraction index of one of the plurality of thesensing fibers changes with an environmental variation, and the changein the refraction index causes an evanescent coupling signal in thesignal fiber corresponding to the laser signal in the one of theplurality of sensing fibers, and the evanescent coupling signal has asame frequency as the laser signal in the one of the plurality ofsensing fibers.

In a third implementation, a method, comprising: transmitting lasersignals to a plurality of sensing fibers in a fiber optic cableassembly, wherein each sensing fiber carries one laser signal, eachlaser signal has a distinct frequency, and the fiber optic cableassembly comprises: an elongate housing; and a signal fiber and theplurality of sensing fibers placed inside the housing and extendinglongitudinally, wherein the plurality of sensing fibers are placedaround the signal fiber, the signal fiber carries one or more evanescentcoupling signals responsive to the laser signals in the plurality ofsensing fibers, and the fiber optic cable assembly is strapped outside atubing in a wellbore formed in a formation; receiving, from the signalfiber of the fiber optic cable assembly, the one or more evanescentcoupling signals; and based on the one or more evanescent couplingsignals, determining characteristics of a down-going seismic signal andan up-going seismic signal.

A first feature, combinable with any of the following features, whereindetermining the characteristics of the down-going seismic signal and theup-going seismic signal includes determining pressure levels of thedown-going seismic signal and the up-going seismic signal based onamplitudes of the one or more evanescent coupling signals.

Thus, particular implementations of the subject matter have beendescribed. Other implementations are within the scope of the followingclaims.

1. A method, comprising: transmitting laser signals to a plurality ofsensing fibers in a fiber optic cable assembly, wherein each sensingfiber carries one laser signal, each laser signal has a distinctfrequency, and the fiber optic cable assembly comprises: an elongatehousing; and a signal fiber and the plurality of sensing fibers placedinside the housing and extending longitudinally, wherein the pluralityof sensing fibers are placed around the signal fiber, the signal fibercarries one or more evanescent coupling signals responsive to the lasersignals in the plurality of sensing fibers, and the fiber optic cableassembly is strapped outside a tubing in a wellbore formed in aformation; receiving, from the signal fiber of the fiber optic cableassembly, the one or more evanescent coupling signals; and based on theone or more evanescent coupling signals, determining flow velocities offirst flowing media flowing through the tubing and second flowing mediaflowing through an annulus between the formation and the tubing.
 2. Themethod of claim 1, wherein the wellbore is a horizontal wellbore, andthe method further comprising using a fiber optic gyro to determine anorientation of the fiber optic cable assembly in the wellbore.
 3. Themethod of claim 1, wherein determining the flow velocities includesdetermining pressure levels of the first flowing media and the secondflowing media based on amplitudes of the one or more evanescent couplingsignals.
 4. The method of claim 1, wherein a distance between the signalfiber and one of the plurality of sensing fibers changes with anenvironmental variation, and the change in the distance causes anevanescent coupling signal in the signal fiber corresponding to thelaser signal in the one of the plurality of sensing fibers, and theevanescent coupling signal has a same frequency as the laser signal inthe one of the plurality of sensing fibers.
 5. The method of claim 1,wherein the environmental variation includes at least one oftemperature, strain-stress, or pressure change around the fiber opticcable assembly.
 6. The method of claim 1, wherein a refraction index ofone of the plurality of the sensing fibers changes with an environmentalvariation, and the change in the refraction index causes an evanescentcoupling signal in the signal fiber corresponding to the laser signal inthe one of the plurality of sensing fibers, and the evanescent couplingsignal has a same frequency as the laser signal in the one of theplurality of sensing fibers.